1. Field of the Invention
The invention relates to a switching power converter and more particularly, to a switching power converter that applies a secondary-side control to achieve system stability and durability.
2. Description of the Prior Art
Switching power converters have the advantages of high working efficiency and limited volume in size, and therefore, are widely used in a variety of electronic devices. FIG. 11 shows the circuit of a conventional flyback converter, including mainly an input circuit 1′ (indicated by a dotted rectangle at the left), an output circuit 2′ (indicated by a dotted rectangle at the right), a transformer T1′ (in between the above two dotted rectangles), and an optical coupler 3′.
Referring to FIG. 11, an input circuit 1′ connects to an input voltage Vin and includes mainly two parts, a transistor Q1′ for switching, and, a controller 11′ for regulating PWM (Pulse Width Modulation). The transistor Q1′ connects to the primary-side coil assembly of the transformer T1′ at one end, and also connects to the output end of the controller 11′ at the other. The input end FB of the controller 11′ connects to one end of the optical coupler 3′.
At the system output side (indicated by a dotted rectangle at the right in FIG. 11) an output circuit 2′ connects to a secondary-side coil assembly of the transformer T1′. The output voltage Vout connects in parallel to one end of the optical coupler 3′, isolating the input circuit 1′ from the output circuit 2′, and conveying the output voltage Vout back to the controller 11′ (of the input circuit 1′). Consequently, the controller 11′ is able to output a more or less stable voltage in correspondence to an output by controlling the on-off states of the transistor T1′.
To sum up, the above-mentioned prior art (illustrated in FIG. 11) makes use of the feed-back control function of an optical coupler to manage the output voltage; the physical characteristics of an optical coupler unavoidably effects the stability and durability of the system. For instance, the coupling efficiency of an optical coupler reflects the accuracy of an output voltage. Furthermore, extra electric elements are required to avoid or reduce the unstable performance of an optical coupler when it is used as a current-stabilizing current charger, adding more cost and bringing in more idleness and worn-out.
FIG. 12 illustrates the electric circuits of another conventional flyback converter, including mainly an input circuit 5′ (indicated by a dotted rectangle on the left-hand side of FIG. 12), a transformer T2′, an output circuit 6′ (as indicated by a dotted rectangle on the right-hand side of FIG. 12). Unlike a conventional converter shown in FIG. 11, the transformer T2′ (shown in FIG. 12) includes three coils: two coils on the primary-side coil assembly (i.e. a primary-side 1st coil assembly, and a primary-side 2nd coil assembly) and one on the secondary-side coil assembly (i.e. a secondary-side 3rd coil assembly).
The input circuit 5′ connects to an input voltage Vin and includes two main parts: a transistor Q2′ for switching, and a controller 51′ for regulating PWM. One end of the transistor Q2′ connects to the primary-side 1st coil assembly N1′ (of the transformer T2′), and the other end, to the output end of the controller 51′. The input end of the controller 51′ connects to the primary-side 2nd coil assembly N2′ (of the transformer T2′).
At the system output side (indicated by a dotted rectangle at the right of FIG. 12), the output circuit 6′ with an output voltage of Vout connects to the secondary-side 3rd coil assembly N3′ (of the transformer T2′). Variations in voltage is conveyed from secondary-side coil assembly (of the transistor T2′) to primary-side coil assembly (of the transformer T2′) and is detected by the controller 51′. As a result, the controller 51′ regulates the output voltage Vout by detecting variations in voltage, and then by controlling the switching on/off of the transistor Q2′.
The above-mentioned conventional circuit has the merit of simplicity in structure, which, nevertheless, relies completely upon the physical characteristics of the transformer T2′ to detect variations in voltage and to regulate the output circuit accordingly. Furthermore, the voltage conveyed back is not continuous, affecting adversely the stability and durability in the electric system, the no-load output voltage, as well as in the efficiency of dynamic voltage management.